C O M P E N S A T I 0 N magnetism of the ship is liable to changes on changing her geographical position, and especially so when steaming at right angles or nearly so to the magnetic meridian, for then sub-permanent magnetism is developed in the hull. Some vessels are more liable to become sub-permanently magnetized than others, and as no corrector has been found for this source of deviation the navigator must determine its amount by observation. Hence, however carefully a compass may be placed and subsequently compensated, the mariner has no safety without constantly observing the bearings of the sun, stars, or distant terrestrial objects, to ascertain its deviation. The results of these observations are entered in a compass journal for future reference when fog or darkness prevails. Every compass and corrector supplied to the ships of the Royal Navy is previously examined in detail at the Compass Observatory established by the Admiralty at Deptford. A trained observer acting under the superintendent of compasses is charged with this important work. The superintendent, who is a naval officer, is also charged with the investigation of the magnetic character of all H.M. ships, to point out the most suitable positions for the compasses when a ship is designed, and subsequently to keep himself informed of their behaviour from the time of the ship’s first trial. A museum containing compasses of various types invented during the 19th century is attached to the Compass Observatory at Deptford. The mariner’s compass during the early part of the 19th century was still a very imperfect instrument, although numerous inventors had tried to improve it. In 1837 the Admiralty Compass Committee was appointed to make a scientific investigation of the subject, and propose a form of compass suitable alike for azimuth, and steering purposes. The committee reported in July 1840, and after minor improvements by the makers the Admiralty compass, the card of which is shown in Figs. 1 and 2, was adopted by the Government. Until 1876, when Sir William Thomson introduced his patent compass, this compass was not only the regulation compass of the Royal Navy, but was largely used in foreign countries in the same or a modified form. The introduction of powerful engines causing serious vibration to compass cards of the Admiralty type, coupled with the prevailing desire for larger cards, the deviation of which could also be more conveniently compensated, led to the gradual introduction of the Thomson compass. Several important points were gained in the latter : the quadrantal deviation could be finally corrected for all latitudes ; frictional error at the cap and pivot was reduced to a minimum, the average weight of the card being 200 grains ; the long free vibrational period of the card was found to be favourable to its steadiness when the vessel was rolling. The first liquid compass used in England was invented by Francis Crow, of Faversham, in 1813. It is said that the idea of a liquid compass was suggested to Crow by the experience of the captain of a coasting vessel whose compass card was oscillating wildly until a sea broke on board filling the compass bowl, when the card became steady. Subsequent improvements were made by Dent, and especially by Ritchie, of Boston, U.S.A. In 1888 the form of liquid compass (Fig. 5) now solely used in torpedo boats and torpedo boat destroyers was introduced. It has also proved to be the most trustworthy compass under the shock of heavy gun lire at present available. The deflector is an instrument designed to enable an observer to reduce the deviations of the compass to an amount not exceeding 2° during fogs, or at any time when bearings of distant objects are not available. It is certain that if the directive forces on the north, east, south, and west points of a compass are equal, there can be no deviation. With the deflector any inequality in the directive force can be detected, and hence the power of equalizing the forces by the usual soft iron and magnet correctors. Several kinds of deflector have been invented, that of Lord Kelvin (Sir William Thomson) being the simplest, but Dr Waghorn’s is also very effective. The use of the deflector is generally confined to experts. The Magnetism of Ships.—In 1814 Flinders first showed (see Flinders’s Voyage, vol. ii. appx. ii.) that the abnormal values of the variation observed in the wood-built ships of his day was due to deviation of the compass caused by the iron in the ship ; that the deviation was zero when the ship’s head was near the north and south points ; that it attained its maximum on the east and west points, and varied as the sine of the azimuth of the ship’s head reckoned from the zero points. He also described a method of correcting deviation by means of a bar of vertical iron so placed
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as to correct the deviation nearly in all latitudes. This bar, now known as a “Flinders bar,” is still in general use. In 1820 Dr Young (see Brande’s Quarterly Journal, 1820) investigated mathematically the magnetism of ships. In 1824 Professor Barlow introduced his correcting plate of soft iron. Trials in certain ships showed that their magnetism consisted partly of hard iron, and the use of the plate was abandoned. In 1835 Captain Johnson, R.N., showed from experiments in the iron steamship Garry Owen that the vessel acted on an external compass as a magnet. In 1838 Airy (Astronomer-Royal) magnetically examined the iron steamship Rainbow at Deptford, and from his mathematical investigations (see Rhil. Trans. R. S., 1839) deduced his method of correcting the compass by permanent magnets and soft iron, giving practical rules for the same in 1840. Airy’s and Flinders’s correctors form the basis of all compass correctors to this day. In 1838 Poisson published his Memoir on the Deviation of the Compass caused by the Iron in a Vessel. In this he gave equations resulting from the hypothesis that the magnetism of a ship is partly due to the permanent magnetism of hard iron and partly to the transientinduced magnetism of soft iron ; that the latter is proportional to the intensity of the inducing force, and that the length of the needle is infinitesimally small compared to the distance of the surrounding iron. From Poisson’s equations Archibald Smith deduced the formulae given in the Admiralty Manual for Deviations of the Compass (1st ed., 1862), a work which has formed the basis of numerous other manuals since published in Great Britain and other countries. In view of the serious difficulties connected with the inclining of every ship, Smith’s formulae for ascertaining and providing for the correction of the heeling error with the ship upright continue to be of great value to safe navigation. In 1855 the Liverpool Compass Committee commenced its work of investigating the magnetism of ships of the mercantile marine, resulting in three reports to the Board of Trade, all of great value, the last being presented in 1861. See articles on Magnetism of Ships and Deviations of the Compass, Phil. Trans. 1839-83, Journal United Service Inst. 18591889, Trans. Nav. Archit. 1860-61-62, Report of Brit. Assoc. 1862, London Quarterly Rev. 1865 ; also Admiralty Manual, edit. 186263-69-93-1900 ; and Towson’s Practical Information on Deviations of the Compass, 1886. (e. w, q ^ Coinpensa.tion.—The term “Compensation” is applied in English law to a number of different forms of legal reparation, e.g., to persons injured by felony, or—under the Riot (Damages) Act, 1886—to persons whose property has been stolen, destroyed, or injured by rioters. It is due, under the Agricultural Holdings Act, 1883, for agricultural improvements; and, under the Workmen’s Compensation Act, 1897, to workmen, in respect of accidents in certain forms of employment. The word is, however, in its strict and most familiar sense, a nomen juris for the reparation or satisfaction made to the owners of property which is taken by the executive government or by local bodies or corporations, under statutory authority, for public purposes. There are two main legal theories on which such appropriation of private property is justified. The American may be taken as a representative illustration of the one, and the English of the other. Though not included in the definition of “eminent domain,” the necessity for compensation is recognized as incidental to that power. The normal procedure is admirably described in Bouvier’s Law Diet., ed. Rawle, 1897, s.v. “Eminent Domain.” According to the law of the United States, there resides in every sovereign State, either as a residuum of the proprietary rights which it creates or protects, or (which is the better opinion) as an incident of its sovereignty, power to take, without reference to the question of compensation, the property of its subjects for purposes of a public character. This power is called “ Eminent Domain.” In English law the only exact analogue to this doctrine is to be found in the prerogative right (which now exists chiefly in theory) of the Crown to expropriate the lands of subjects for the purposes of the defence of the realm (see Attorney-General v. Tomline, 1879, 12 Ch. D. 214). With that exception, the rule of English constitutional law is that the property of the citizen cannot be seized for purposes which are really “ public ” without a fair pecuniary
